1. Field
The embodiments relate to an OFDM receiver apparatus for receiving OFDM signals by using a plurality of receiver circuits and OFDM receiving method.
2. Description of the Related Art
In digital modulation using a single carrier wave (hereinafter referred to as the carrier), a symbol period becomes shorter as the transfer rate becomes higher. For that reason, signal demodulation may be difficult under multipath environments. Note that a term multipath, in general, means an environment in which radio waves transmitted from a transmitter station reaches a receiver station via plural paths, and this is created by, for example, reflection at obstacles. An environment similar to the multipath is created in communications systems in which radio waves carrying the same signal from plural transmitter stations are simultaneously transmitted. Therefore, the “multipath environment” used in the following description includes the above two environments.
Orthogonal Frequency Division Multiplexing (OFDM) is proposed as one of transmission systems that are intended to improve reception performance under the multipath environment. In OFDM, data is transmitted by utilizing plural carriers orthogonal to each other on a frequency axis. For that reason, a symbol period of data transmitted by each carrier becomes longer and therefore reception performance degradation is smaller even under a multipath environment with large delays. In addition, modulation methods can be changed for each carrier in OFDM.
A modulation employing IFFT (Inverse Fast Fourier Transform) and a demodulation employing FFT (Fast Fourier Transform) are performed in OFDM. For that reason, OFDM has high use efficiency of frequency, and application to digital terrestrial broadcasting has been widely discussed. In Japan, OFDM has been employed in ISDB-T (Integrated Services Digital Broadcasting-Terrestrial), one of the digital terrestrial broadcasting standards.
FIG. 1 is a diagram illustrating a configuration of a conventional OFDM receiver apparatus. The OFDM receiver apparatus receives and demodulates OFDM signals. The OFDM signals transmit data using plural carriers, as shown in FIG. 2. In an example shown in FIG. 2, scattered pilot (SP) signals are allocated at certain frequency intervals. In the ISDB-T, 432 carrier waves are multiplexed, and the SP signals are allocated at every 12-carrier cycles on a frequency axis and are allocated at every 4-symbol cycles on a time axis.
The OFDM signals are received by a tuner 101 and converted into digital signals by an A/D converter 102. An orthogonal demodulator 103 generates orthogonal signals (I-component signal and Q-component signal) from the digital signals obtained in the A/D converter 102. An FFT unit 104 converts time-domain signals into frequency-domain signals by executing FFT operation for each symbol. A transmission path equalizer 105 corrects phase rotation that occurred in the transmission paths. An error correction unit 106 executes error correction and regenerates transmission data.
An IFFT unit 107 converts the frequency-domain signals output from the FFT unit 104 into time-domain signals. A delay information extraction unit 108 generates a delay profile serving as delay information based on the time-domain signals output from the IFFT unit 107. The delay profile represents changes in reception power on time axis. The delay information extraction unit 108 generates FFT window control instruction that is for instructing the position of an FFT window (i.e. FFT start timing) based on the delay profile and provides the instruction to the FFT unit 104. The FFT unit 104 executes FFT operation for each symbol according to the FFT window control instruction.
In order to improve reception performance under the multipath environment, OFDM further introduces guard intervals. The guard intervals are explained in the following descriptions with reference to FIG. 3A and FIG. 3B. In FIG. 3A and FIG. 3B, under a multipath environment in which a main wave (a desired wave) and its delay wave (an interfering wave or an undesired wave) are present, the FFT operation on a symbol n of a received OFDM signal is to be executed.
The FFT operation is executed by information in the FFT window set on the time axis being input to the FFT unit 104. The width of the FFT window corresponds to one symbol time. Assume that, at that time, the guard intervals are not inserted between the symbols. Then, as shown in FIG. 3A, when retrieving the information of the symbol n in the main wave, not only the information of the symbol n in the delay wave but also the information of the symbol n−1 of the delay wave is retrieved. In other words, data of the symbol n is regenerated based on the information of the symbol n and the information of the symbol n−1. As a result, inter-symbol interference occurs and reception quality is degraded.
In view of this problem, OFDM has guard intervals inserted between symbols as shown in FIG. 3B. A guard interval i (i is a number for identifying each symbol) is obtained by copying the information in the end of the symbol i. Note that in the mode 3 of the ISDB-T, the guard interval is 1/8 symbol period.
As shown in FIG. 3B, if the FTT window is set at the symbol timing of the main wave, the information of the symbol n in the delay wave and the information of the guard interval n of the delay wave are also retrieved when retrieving the information of the symbol n in the main wave. However, the information of the guard interval n is obtained by copying a part of the information of the symbol n. In this case, therefore, the FFT operation is executed only on the information of the symbol n. As a result, the inter-symbol interference does not occur and therefore the reception quality is improved.
Depending on the communications system configurations, OFDM receiver apparatus may receive main wave and its preceding waves. The preceding waves may be present in systems such as SFN (Signal Frequency Network) in which the same signals are transmitted from plural transmitter stations simultaneously.
When the preceding waves are present, as shown in FIG. 4A, if the FFT window is controlled at the symbol timing of the main wave, inter-symbol interference occurs. In other words, when retrieving the information of the symbol n in the main wave, not only the information of the symbol n in the preceding wave but also the information of the symbol n+1 in the preceding wave is retrieved. For that reason, when the preceding wave is present, as shown in FIG. 4B, the FFT window is controlled at the symbol timing of the preceding wave. By doing so, obtained demodulated data is based only on the information of the target symbol.
In addition, diversity reception is known as one of technologies to improve the communication quality, although its application is not limited to OFDM. In diversity reception, the same signals are received using plural receiver circuits. In selection diversity, a signal with the best communication quality is selected and output. In combining diversity, plural received signals are combined and transmission data is regenerated from the combined signals.
It should be noted that Patent Document 1 (Japanese Patent Application Publication No. 2003-229833) discloses a technology for controlling FFT windows in a diversity reception circuit. An FFT common window period calculation unit calculates an FFT common window period in which inter-symbol interference of an OFDM signal included in two or more antenna signals becomes minimum.
Patent Document 2 (Japanese Patent Application Publication No. 2005-150935) discloses a configuration for switching a master branch and a slave branch in a diversity receiver apparatus.
Patent Document 3 (Japanese Patent Application Publication No. 2006-229323) discloses a configuration for setting the position of an FFT window based on delay profile signals and signals indicating presence/absence of ghost.
In OFDM, as described above, by inserting a guard interval between symbols, degradation of the reception quality is reduced under a multipath environment. However, even if guard intervals are provided, inter-symbol interference cannot be prevented under an environment in which multipath delay is larger than the guard intervals, and the communication quality is deteriorated. However, depending on the modulation methods (e.g. in a case of a low multilevel modulation such as BPSK or QPSK), even if the multipath delay is larger than the guard intervals, signals can be received as long as the position of the FFT window is set properly. Therefore, it is important to properly set the position of FFT windows. Note that in the following description, the term “multipath delay” includes “time difference between preceding wave and main wave” and “time difference between main wave and delay wave”.
Control of FFT windows in the configuration shown in FIG. 1 is performed as described below. SP signals are extracted from output signals of the FFT unit 104. The IFFT unit 107 obtains time-domain signals by executing the IFFT operation on the SP signals. The delay information extraction unit 108 generates a delay profile from the time-domain signals. If the preceding waves are not present, as shown in FIG. 3A and FIG. 3B, an FFT window is set at the symbol timing of the main wave. Meanwhile, if the preceding wave is present, as shown in FIG. 4B, an FFT window is set at the symbol timing of the preceding wave. As described above, control of FFT windows differs depending on whether the preceding wave is present or not.
The SP signals used for generating the delay profile, however, are inserted at three-carrier intervals in the example shown in FIG. 2. In such a case, detectable delay (i.e. a range in which FFT/IFFT operations can be executed) is 1/3 symbol period. Given that one symbol period is 1.008 ms (mode 3 of the ISDB-T), spreading of delay would be 336 μs. In consideration that both of the preceding waves and delay waves are present, detectable delay is ±1/6 symbol period (±168 μs).
Assume that, as shown in FIG. 5A, 200-μs delay wave is present. The detectable delay here is ±168 μs. The delay wave, then, is detected as “preceding wave of 136 μs” in the delay profile obtained from the operation result of the IFFT unit 108 as shown in FIG. 5B due to the FFT/IFFT characteristics. In other words, in this case, whether a 200-μs delay wave is present or a 136-μs delay wave is present cannot be identified. As a result, although a 200-μs delay wave is present in reality, if the position of FFT windows is determined assuming that a 136-μs preceding wave is present, communication quality would be deteriorated by inter-symbol interference.
As explained above, because the position of FFT windows cannot be determined properly, communication quality may be deteriorated when the multipath delay is large.